"These guys are making a list and checking it twice. They remember every tiny little slight, everything that was ever said to them or done to them," the psychologist said. "So it's accumulating, and it hits this critical mass at some point and sets this violent trajectory in motion."

Shooter Jared Padgett, 15, was passionate about guns and had a temper, students have said. But he also seemed like a nice, normal kid.

Emilio Hoffman, 14, died June 10, 2014, the victim of a shooting at Reynolds High School in Troutdale, Ore.(Photo: Troutdale, Ore., Police Department)

Because something unknown can set off a teen gunman, any victims often are not connected to the shooter and may be simply in the wrong place at the wrong time, Colistro said. Police have said that they have not been able to establish a link between two freshmen at Reynolds High, Padgett and his victim, Emilio Hoffman, 14.

More than 450 seniors will graduate as planned Thursday night from Reynolds High; they had a rehearsal in the morning at Portland's Veterans Memorial Coliseum. But also as planned, they won't be back at the school where many on Wednesday picked up backpacks and other belongings that they had to drop as they evacuated the building the day before.

Still undecided is exactly how the school year will end for the rest of the students. They were in the middle of final exams when the shooting occurred, and the school is the second largest in the state with 2,800 teens.

Even though Tuesday's shooting ended with Padgett's suicide, copycat gunmen are a significant danger, Colistro said.

Before Reynolds High's incident, another school shooting had happened just five days previous when a man killed a student and injured three others June 5 at Seattle Pacific University, fewer than 200 miles north of Troutdale.

Shooters often begin with daydreaming, move to planning, then begin acquiring weapons, practicing and acting out. Colistro compares it to a staircase. The top brings explosive action.

"So right now, there are countless kids on that staircase at various levels," he said. "And there are a few on the top step and they're gonna see this and they're gonna go, 'Time to act."

A woman who didn't want to be identified brought a stack of T-shirts on June 11, 2014, to Reynolds High School in Troutdale, Ore., to honor shooting victim Emilio Hoffman, 14.(Photo: Randy L. Rasmussen, AP)